Conventional methods of producing metal matrix composite (MMC) parts can be categorized into two main areas, powder metallurgy and casting. Powder metallurgy includes techniques such as hot isostatic pressing, vacuum hot pressing, and direct extrusion. Casting techniques generally involve pressure or pressureless casting of a molten metal into a preformed die to shape the molten metal accordingly. All of these procedures require expensive tools and machining, and thus are generally unattractive for producing a small quantity of parts.
When only a small quantity of replacement parts or prototypes are desired, subtractive methods typically are employed. Subtractive methods teach subtracting material from a starting block to produce a more complex shape. Examples include milling, grinding, drilling, and lathe cutting. Conventional subtractive methods are deficient because they produce a large amount of waste material for disposal, involve a large initial expense in setting up the tooling, and result in significant tool wear which increases operating costs. Furthermore, such methods cannot produce parts with unique shapes or complicated internal formations.
Other conventional processes are additive, such as welding, plating, and cladding. Such processes are generally limited to coating or depositing material on a starting article. More recently lasers have been used to build a part under computer aided design/ computer aided manufacturing (CAD/CAM) control, where a laser is directed at a surface and powder flows to the surface from a hopper via a powder feed device. The laser builds the part in layers as it heats, melts, and shapes the part in a desired configuration. Such a method is disclosed in U.S. Pat. No. 5,156,697 to Bourell et al. (Bourell). Bourell is hereby incorporated by reference in the present application. U.S. Pat. No. 6,046,426 to Jeantette, U.S. Pat. No. 5,993,554 to Keicher et al., and U.S. Pat. No. 5,961,862 to Lewis et al. also describe a process for making unreinforced metal parts using the process described herein for making MMC parts.
Bourell is directed to producing parts from either powders mixed together, such as mixed copper and tin particles, or coated powder particles, such as iron or steel coated with poly methyl methacrylate (see Column 8, lines 29-60). The disclosure for mixed particles is limited to two materials which have different bonding or disassociation temperatures (see Column 8, lines 61-66). Applications for coated powder particles are limited due to the prohibitive cost of such particles. Coated materials typically require metal or ceramic particles to be sprayed with a coating over the particles and are very expensive to produce.
Parts produced by the method of Bourell exhibit a higher melting point than the constituent materials of the powder, but offer no other notable property enhancements.
Titanium based alloys (i.e. alloys comprising a mixture of titanium and at least one other metal, such as Ti-6Al-4V) have been used as a matrix with a reinforcement material to increase strength and modulus. The matrix can be formed by the conventional technique of cladding, where a layer of reinforcement material is deposited on a layer of metal alloy particles. Such techniques are disclosed in U.S. Pat. No. 4,906,430 to Abkowitz et al. where the materials are pressed and sintered together, and U.S. Pat. No. 4,968,348 to Abkowitz et al., where TiB2 is the reinforcement material formed by blending and sintering the material. U.S. Pat. No. 4,731,115 to Abkowitz et al. discloses a cladding process where TiC serves as the reinforcement formed by blending, pressing, and sintering the material.
Under the conventional cladding process as taught in the Abkowitz patents, the metal alloy material and reinforcement material are blended together and then pressed and sintered in a mold or die at elevated temperature to form discontinuously reinforced metals (DRX, where X represents the metal). DRX composite materials are characterized by a metal matrix with a reinforcement interspersed in the matrix. Such composite structures have been produced using conventional powder metallurgy and casting techniques as described in the Abkowitz patents, but have not been successfully produced using laser processing.
The article titled “Evaluation of a Discontinuously Reinforced Ti-6Al-4V Composite” by Yolton et al. (Yolton) discloses a process of synthesizing an in-situ Ti-6Al-4V composition containing additions of boron and/or carbon to form TiB and/or TiC reinforcements. Yolton obtains a fine distribution of TiB and TiC reinforcements through conventional processes of hot pressing followed by extrusion of the hot pressed material. In the hot pressing process, a structure is fabricated from Ti-6Al-4V and reinforcement material which could be formed in a mold or in a sheet or block. Thereafter, an extrusion process is carried out in which the shape of the hot pressed structure is destroyed. Extrusion involves thermal-mechanically working the structure, which changes the shape and causes the reinforcements to align in the direction of extrusion. In the final structure, Yolton did not achieve a desired random orientation of reinforcement particles. And further, near net shaped structures or parts cannot be made without taking additional steps to mechanically work the hot pressed structure.